As known, an either straight or curved “manual” treadmill is a motorless exercise machine which can be manually actuated by the user through the interaction of the lower limbs with the walking/running belt. In other words, a “manual” treadmill does not have a motor.
A straight or curved manual treadmill typically comprises a frame extending along a longitudinal development direction parallel to the user's advancement direction while walking or running.
Moreover, such a manual treadmill comprises a first front rotational shaft and a second rear rotational shaft about which a walking/running belt is wound.
In the case of a curved manual treadmill, the user's walking/running belt is typically mounted on the first front rotational shaft and on the second rear rotational shaft so as to have a curved side profile along, and with respect to, the longitudinal development direction of the frame on the part facing upwards, i.e. having a first descending portion starting from the first front rotational shaft and a second portion, opposite to the first portion, ascending towards the second rear rotational shaft.
While the user runs or walks on the walking/running belt, the weight force exerted by the user at the first descending portion of the walking/running belt allows the potential energy to be transformed into kinetic energy and thus the rotation of the walking/running belt from the first front rotational shaft to the second rear rotational shaft to be generated only by means of the interaction of the user's lower limbs with the walking/running belt.
In order to ensure the rotation of the walking/running belt only by means of the interaction of the user's lower limbs, it is desirable to have a walking/running belt which keeps the curved side profile as much as possible with respect to the longitudinal development of the base.
Several technical solutions exist today to address this issue.
In a first technical solution of the prior art, the frame of the manual treadmill is provided with corresponding side guides closed along the entire curved side profile of the walking/running belt. The walking/running belt is provided on both sides with corresponding bearings inserted and suitable to roll within the side guides of the manual treadmill frame.
Such a solution has drawbacks related to the excessive friction of the bearings when they roll within the side guides, resulting in a greater sliding resistance with a consequential reduction in the manual treadmill's efficiency and an increase of its noise. Furthermore, there is a problem of tolerances between each side guide and the walking/running belt, which must have some clearance. Again, such a solution has assembly drawbacks including the difficulty of keeping the correct center to center distances between the center of the curved side profile of each side guide, the first front rotational shaft and the second rear rotational shaft.
In another technical solution of the prior art, instead, the manual treadmill is provided with a so-called synchronization belt between the first front rotational shaft and the second rear rotational shaft, suitable to ensure the synchronized rotation of the first front rotational shaft and of the second rear rotational shaft during the rotation of the walking/running belt.
However, this solution also has the disadvantage related to the friction generated by the rolling of a belt.
Furthermore, this solution suffers from increased noise due to the meshing of the teeth, specifically if the synchronization belt is toothed. Furthermore, when a braking action is applied, the elasticity of the synchronization belt causes a tensioning of the walking/running belt on the side facing upwards.
This causes the walking/running belt to lift from the side support rollers, resulting in the knocking of the walking/running belt on the side support rollers at the user's each step.